Prior art automatic power control circuits are typically constructed to input a tiny known fraction of the output signal from a power amplifier and compare its average amplitude against a predetermined reference value corresponding to the desired power level. A variable attenuator is provided in front of the power amplifier input to regulate the input signal level and is adjusted based on the result of such comparison. By using negative feedback to reduce or increase the attenuation of the signal input to the power amplifier, such a control circuit regulates the level of amplifier's output signal to maintain a predetermined power level.
Conventionally, the actual circuit used to realize this automatic control differs according to the type of radio signal being amplified. For example, in satellite radio communications, there are two main types of multichannel signal transmission formats. One is multiple carrier multiplex transmission, in which a single transmitter sends a plurality of carrier waves, each corresponding to a different channel. The other is packet transmission, in which signals are bundled in packets which are transmitted intermittently in bursts.
With multiple carrier transmission, because a single power amplifier amplifies a plurality of different carrier waves (common amplification), care must be taken to prevent cross modulation between the different carriers (channels). For example, NEC Corporation's Japanese Laid Open Patent Application Publication 143,624/82, entitled "Automatic Electrical Power Control Circuit," discloses a control circuit for a multiple carrier transmission power amplifier. This control circuit is constructed to have the desired output power level indicated by a control signal input from a source external to the power amplifier control circuit itself. This control circuit has a negative feedback loop in which the externally provided digital control signal is input to a digital-to-analog (D/A) converter to create an analog reference voltage.
A predetermined tiny fraction of the A.C. output from the power amplifier is rectified (detected) to derive a low frequency measured output signal. Then the difference between the measured output signal and the reference voltage is amplified by an analog differential amplifier. The amplified difference is then used as a correction signal to negative feedback control a variable attenuator regulating the signal input to the power amplifier.
But such a circuit would not work well with packet transmission, since the transmitted packet signal is generated in periodic or irregular intermittent bursts separated by intervals of no transmission signal. The "no signal" intervals would mislead such a power control into greatly reducing the variable attenuation, resulting in overamplification when the next transmission burst begins.
Instead, for packet signals the control circuit needs a HOLD circuit in the negative feedback loop to provide stability. For example, NEC Corporation's Japanese Laid Open Patent Application Publication 173,507/86, entitled "Automatic Level Control Circuit," shows a control circuit constructed from an analog comparator 6, which compares a local analog reference voltage 7 with a detected known fraction of the amplifier output, an up/down counter 8 which has a HOLD function, and a digital-to analog (D/A) converter 12 which converts the up/down counter's digital output into an analog signal to control a variable attenuator 13 regulating the input to the power amplifier. When the transmission signal is present, a clock pulse causes the up/down counter to count up or down depending on the output from the analog comparator 6. The D/A converter then regularly converts the changing counted value in the digital up/down counter into an analog correction signal suitable for controlling the variable attenuator.
If no transmission signal is present at the input, the clock pulse ceases and the up/down counter holds the count value which existed just prior to the loss of input signal. This retained digital count value is supplied to the D/A converter, which produces an analog signal for controlling the attenuator. Note that this circuit is not provided with an input terminal for an external power control signal.
Since satellite radio communications are generally adversely attenuated by rain, an automatic transmission control circuit for such applications must be able to automatically compensate for rain attenuation, regardless of whether the signal transmission format is multiple-carrier transmission or packet transmission. In other words, it is highly desirable for the control circuit to be able to set the output power level in response to an external signal, such as one indicating the onset of rain.
In recent years there has been a growing demand for small, low-cost earth stations for satellite communication (hereafter "small ground stations"). The equipment used in such small ground stations must be compact and economical. Because generally such stations use the packet transmission format, an automatic transmission power control circuit for them must not only be compact and economical. It must also operate stably when transmitting the burst signals of packet transmission and provide a terminal for an external power control signal.
Circuit designs using analog comparators along the lines of the previously mentioned prior art circuits have the disadvantage that they generally have to be constructed from a number of individual parts. Therefore, such circuits cannot easily be miniaturized and made less expensive.